"Sist" or "I"

Norwegian 4:
“Sist gang” meaning “last time.”
“I fjor” meaning “last year.”

Is it wrong to say “Sist fjor” meaning “last year?” Also, what is wrong with the word “Ar” for “year?” Is “I ar” or Sist ar", wrong?

Yes, it’s wrong. It’s because “(i) fjor” means “previous/last year”. It’s similar to “yesterday” in English. We don’t say “last day” to mean “yesterday”. The same is the case for Norwegian. We say “i fjor”, not “siste år”.